This application claims foreign priority under 35 U.S.C. xc2xa7119 of EPO patent application 00204266, filed Nov. 30, 2000, incorporated herein by reference.
The present invention relates to a process for the esterification of organic acids with alcohols in a reactor of the chromatographic type in which the heterogeneous solid phase is capable of acting both as an esterification catalyst and as a means exhibiting preferential adsorption towards one of the reaction products (typically water). The process to which the present invention relates is particularly improved compared with conventional technology owing to the use, as the means for regenerating the catalyst, of a desorbent to which has been added a second compound which, by chemical reaction, completes the removal of the adsorbed water.
Esterification reactions are limited by a chemical equilibrium and therefore techniques for the industrial preparation of an ester involve two consecutive steps. The first is the reaction itself, which finishes when the conditions of equilibrium have been reached. A second step is therefore necessary in order to separate the products of interest from the mixture containing products and reagents. The disadvantages of that technique are of an economic nature because, apart from the energy costs involved in the separation operations, it involves the use of different reactors and separators.
A more advantageous technique resides in combining those two steps in a single reaction and separation apparatus. This can be carried out either in a continuous or in a semi-continuous unit, using various separating techniques, such as: evaporation, distillation, chromatography or the use of membranes. U.S. Pat. No. 4,435,595 describes, for example, the use of reactive distillation to produce high-purity methyl acetate, by esterifying methanol and acetic acid. The process provides for the use of acetic acid and methanol which flow in counter-current through a single reactive distillation column in the presence of an acid catalyst (for example sulphuric acid).
The acetic acid, in addition to being a reagent, also acts as an extracting agent for the non-reacted methanol and for the water which has been produced. Therefore, the methyl acetate is separated continuously from the acetic acid and removed from the top of the column.
Using reactive distillation, it is therefore possible to take the reaction beyond the equilibrium conversion value. One of the major disadvantages of that technique is the cost of the reactive distillation column. Owing to the fact that sulphuric acid is used as the reaction catalyst, the column has to be constructed using a suitable material that is not subject to corrosive phenomena, and therefore using expensive materials.
Reactive chromatography is described as a technique employing a chromatographic system which is used both to convert one or more components and, at the same time, to separate one or more of the products which have been formed. Sardin and Villermaux (Nuov. J. Chim. 1979, 3, 255) have proposed the reaction of acetic acid and ethyl alcohol to form ethyl acetate and water and the simultaneous separation of the products in a fixed-bed chromatographic reactor operating in discontinuous mode. The solid contained in the fixed bed is a mixture of activated alumina for effecting separation and cationic resins in acid form for catalysing the esterification reaction. The ethyl acetate produced is not adsorbed by the bed, while the water is; this enables the two products to be separated. The system operates in a pulsed manner and exhibits a conversion greater than that which can be produced at equilibrium.
Mazzotti et al. (Chem. Eng. Sci. 1996, 51 1827 and Ind. Eng. Chem. Res. 1997, 36,3) propose the use of ion exchange resins in acid form in a discontinuous chromatographic reactor for the esterification reaction between acetic acid and ethyl alcohol. Apart from catalytic capabilities, the resins also exhibit selective adsorption properties suitable for promoting the separation of the reaction products. Analogously (U.S. Pat. No. 5,405,992) a process is proposed which uses a S.M.B.R. (Simulated Moving Bed Reactor) for esterification between an alcohol and a carboxylic acid. The proposed system acts as a catalyst for esterification and as an adsorbent for only one of the products. The adsorbed compound is subsequently regenerated by means of a suitable desorbent. The system is filled with macroreticular ion-exchange resin in acid form, which is efficient both as a catalyst and as a selective adsorbent for water.
That technology is limited by the regeneration stage and by the subsequent separation operation for recovering the desorbent. It has been ascertained several times that the enormous amount of desorbent, which is indispensable for returning the resin bed to the initial conditions (that is to say, in order to desorb all of the water) makes the process economically disadvantageous.